> I need to be able to create a Database on a remote server from inside the Inno Setup application (an installer written in Pascal) and I do not want to use an SQL Script to do it (for reasons I won't go into here).

Ok, write your own application which do creation and put all SQL into it.

> I know I need to get the SYSDBA Username and Password from the user, plus the IP Address of the FB Server, & I realise that the FB Client needs to have already been installed.

Not only SYSDBA can create databases. Actually, it is not recommended
to create database and objects inside from SYSDBA name.

SY, SD.